Metal halide discharge lamps usually have a discharge vessel or discharge bulb made of quartz glass. In order to improve the color rendition, higher operating temperatures are necessary than the glass vessels can safely accept, and discharge vessels made of ceramic material have been proposed. Typical operating power ratings are between 100 to 250 W. The discharge vessels are generally elongated, for example cylindrical, and closed off at their open ends with plugs. The plugs have a central bore or opening through which current supply leads pass. The electrodes are then connected to the current supply leads at the inside of the vessel, and external supply connections are made at the outside. The discharge vessel may be surrounded by an outer envelope, through which the external current connections are carried, for example via molybdenum foils embedded in pinch seals.
Ceramic discharge vessels are known, and technology to close off the discharge vessels and seal plugs therein likewise is known. Sodium high-pressure discharge lamps typically have such structures. Usually, tubular or rod or pin-like through-connections pass through the plugs. The through-connections are, generally, made of niobium, see British Patent 1,465,212 and European 0 034 113. The connections, that is, the tubes or pins or rods, are melt-sealed in the ceramic plugs by a glass melt or glass solder, or by a melt ceramic technology.
Unfortunately, melt connections known and used in sodium high-pressure discharge lamps cannot be used in metal halide discharge lamps. The lifetime of such lamps is substantially decreased when such melt connections are used, since the metal halide fill has the tendency to corrode the melt ceramic used as a seal as well as the niobium lead-through.
The referenced U.S. Pat. No. 4,545,799, Rhodes et al, describes a sodium high-pressure lamp which uses a niobium tube as a lead-through. The niobium lead-through is directly sintered into the ceramic discharge vessel without melt ceramic by using, originally, a "green" Al.sub.2 O.sub.3 ceramic. This is possible since both materials, that is the aluminum oxide ceramic and the niobium, have roughly the same thermal coefficient of expansion (TCE), in the order of 8.times.10.sup.-6 /K. This improves the lifetime. The problem of corrosion of the niobium when a metal halide fill is used, however, remains so that this technology cannot be transferred to metal halide discharge lamps.